The present invention relates to the decoding or demodulating of binary signals, and in particular signals that convey the binary information by means of frequency modulation, for example by encoding in a Manchester type format. More specifically, the present invention relates to the decoding of two or more binary signals that can be read at different and varying speeds, for example time codes that are read from two or more variable velocity tape recorders.
In the recording of information on magnetic tape, it is often necessary, and desirable, to record separate pieces of information on separate tapes that are subsequently played back together to reproduce the recorded information. For example, in the recording of a live performance, the visual information might be recorded by means of a videotape recorder while the accompanying audio information is recorded with an audio tape recorder. During playback, it is necessary to insure that the two tapes on which the information is recorded are in synchronism, to accurately reproduce the information as recorded. In order to effect such synchronism, a time code is typically recorded on each tape along with the visual or audio information of interest. The time code basically indicates the portion of the tape that is being presented to the magnetic writing and reading heads of the recorder.
The time code information generally comprises a binary signal that is recorded on the magnetic tapes. One type of binary coding format that has been found to be quite suitable for use in this context is a format that is effectively a frequency modulation technique known as Manchester coding. In formats within this family of codes, the binary signal comprises a series of bit cells, each of which contains one bit of binary information. Each bit cell in the signal is distinguished from the immediately preceding bit cell by a transition in the signal, i.e. a switching from one voltage level to another. The binary information itself is conveyed by means of the presence or absence of a transition within each bit cell. For example, a transition occurring approximately near the middle of a bit cell can be indicative of the true, or binary one state, and the absence of a transition can be indicative of the other binary state, i.e. false or binary zero.
Other codes within this same general format may convey the binary information by means of the placement of the transition within the bit cell, e.g. within the first one-third of the bit cell to convey one binary state and within the last one-third to indicate the other state. Still other codes utilize selective suppression of transitions within the signal to increase storage density. Examples of this latter type of code, known as the Miller code, and derivatives thereof are disclosed in U.S. Pat. Nos. 3,108,261; 4,027,335 and 4,234,897.
The advantages of using this type of coding format for the time code on magnetic recording tape lie in the fact that it is a self-clocking signal and that the D.C. content of the signal is not relied upon to convey the binary state information. More particularly, since each bit cell within the signal is distinguished from the others by means of a transition in the signal, there is no need to provide a separate clocking signal that indicates when each piece of binary information is being presented. Furthermore, since it is only necessary to determine whether or where a transition occurs within a bit cell to derive the binary state information from the signal, the D.C. content of the signal, i.e. the absolute voltage level of the signal, is not a critical factor. This latter consideration becomes particularly significant in magnetic tape recording environments, since the characteristics of magnetic tape are such that the actual voltage level of a signal may be frequency dependent, and hence subject to change as the transition rate varies. Even a reversal of the polarity of the signal does not affect the demodulation process with this type of signal.
Generally, it is a relatively simple task to read a time code that is indicated by a binary signal in this type of code format, to thereby determine the position of the tape. However, reading can present a difficult problem when the tape undergoes changes in speed, which can occur quite frequently when one tape is being run at a speed dependent on the position of another. More particularly, the problem in reading this format is occasioned by the fact that the length, i.e. time duration, of each bit cell varies as the speed of the tape is varied, and the signal is essentially asynchronous. For example, when the tape is running at one particular speed, each bit cell might have a duration of one millisecond. Therefore, to determine the binary information contained in the signal it is only necessary to determine whether or where transitions occur in the signal within less than one millisecond of each other. However, if the tape is then slowed down to one half its previous speed, each bit cell will have a duration of two milliseconds and therefore a transition occurring within a bit cell might not be detected if one is looking for transitions that occur within less than one millisecond of each other. Conversely, if the tape speeds up to a point where each bit cell has a duration of 1/2 millisecond or less, then all transitions will occur within less than one millisecond of each other and the resulting information that is obtained will be erroneous.
Accordingly, in order to successfully detect a binary time code that is encoded in a frequency modulation type format, it is necessary to be able to establish the length of a bit cell within the binary signal, and to be able to adjust the length of the established bit cell in accordance with variations in speed of the magnetic tape to thereby properly detect transitions occurring within bit cells.
One type of demodulator for decoding a Manchester format time code recorded on a variable speed magnetic tape is disclosed in U.S. Pat. No. 4,040,100. The operation of that demodulator is based upon the comparison of the intervals between successive transitions in the time code signal to determine one of three states related to the information in the signal. The principle of operation of this system provides a reliable method for decoding a time code signal, and it is desirable to improve upon the concepts disclosed in that patent. More particularly, while the system disclosed in the patent is successful, it is not without attendant limitations. Foremost among these limitations is the fact that a relatively high number of integrated circuit chips are required to produce a practical implementation of the system. This requirement is due to the high number of logic functions performed by the circuit. In addition, a separate demodulator is required to decode each different time code signal, i.e. there is a one-to-one relationship between demodulators and recorders, because of relatively slow processing times.
Furthermore, the comparison of successive time intervals in the manner disclosed in the patent can result in decoding errors when certain types of noise or disturbances affect the time code signal. For example, a skewing of the tape on which the time code is recorded can result in a transition being detected relatively near the end of a bit cell, rather than near its middle. In such a case, one pulse in the bit cell will appear to be twice as long as the other pulse, and could result in an erroneous indication since they should be detected as pulses of approximately the same length.